What exactly do axial motion metal expansion joints solve?
The thermal expansion and contraction of pipes is learned in physics class. The formula is just one line:Δ L = α L Δ T。 But you would be naive to think that a formula would pick out the inflation excerpt.
In actual engineering, which one does not mix the direction of the pipe system, the position of the fixed bracket and the spacing of the guide frame? For example, a DN300 steam pipe with a length of 50 meters, the temperature rises from 20℃ to 300℃, the linear expansion coefficient of carbon steel is calculated as 0.012 mm/ (m·℃), and the elongation is 168 mm. This is without counting the thrust generated by the internal pressure of the pipe. If you deadlock both ends with fixed brackets, it is a trivial matter for the pipeline stress to exceed the standard, but it is really troublesome for the pipe frame to deform and the equipment interface to crack.
At this time, we have to rely onAxially moving metal expansion jointTo absorb this displacement. Its core logic is: let the bellows deform in the axial direction and eat up the thermal displacement, rather than let the pipe carry it hard.
Then why can't you use a transverse or hinged hardtop? The reason is simple – in the wrong direction. The transverse expansion joint is compensated by transverse displacement. If you use it to absorb axial displacement, the force state of the bellows will be completely wrong, ranging from a sudden reduction in life to instability and bursting. Not to mention the hinge type, it can only absorb angular displacement. If you insist on it expanding and contracting axially, that is not a type selection, it is looking for death.
What is the difference between three common axial structures?
There are three mainstream axial corrugated expansion joints on the market:Universal corrugated expansion joint、External pressure single axial expansion joint、Straight pipe pressure balanced expansion joint。
Universal corrugated expansion jointThe structure is the simplest, the bellows is directly connected to both ends of the pipeline, and the price is cheap. It is the first choice for many low-pressure and normal-temperature pipelines. But it has a flaw-high internal pressure thrust. Medium pressure acting on the bellows creates a blind plate force, which requires a fixed bracket or main fixing point to withstand. As soon as the pressure is high and the pipe diameter is large, the fixed bracket has to be made like a bunker, and the civil construction cost will rise.
AgainExternal pressure single axial expansion jointIt moves the bellows to the outside, and the medium goes inside the guide tube without directly contacting the inner wall of the bellows. There are two advantages: First, the stability is good, and the bellows is not prone to column instability under external pressure; Second, it has high pressure resistance, especially suitable for high-pressure steam pipelines. I have seen this one used on the main steam pipeline of many power plants. The pressure is above 10MPa and the temperature is about 540℃, so it can still hold up. The price is a bit higher than the general purpose model, but when you use it in the right place, you can't save this money.
And the most expensiveStraight pipe pressure balanced expansion joint。 You can tell by the name of this thing-it cancels out the internal pressure thrust through a set of balanced bellows, preventing the blind plate force from transmitting to the equipment. Where is it needed? Steam turbine import and export, compressor pipeline, these equipment are very valuable. Do you let them bear tens of tons of blind plate force? Once the interface is deformed, the maintenance cost is enough to buy several expansion joints.
Three structures, each with its own boundaries. Don't use the general-purpose type to top the high-pressure steam, and don't put the external pressure single-type scheme on the steam turbine pipe system to save money-buying the wrong one is more expensive than buying the expensive one.
Deflectors, tie rods and nuts, the details hide accidents
Many people choose expansion joints, only looking at the material and wavenumber of bellows, and accessories such as guide tubes and tie rods don't care at all. You have a long memory when something happens to you.
guide tubeInstalled inside the bellows, the function is to reduce the direct erosion and wear of the media on the inner wall of the bellows. But there are two things about this thing: direction and clearance. The direction is reversed, and the medium is directly poured into the wave trough of the bellows, and the erosion and wear wear wear through the wave wall in minutes. This accident really happened. The gap is not enough, the throttle tube and the inner wall of the pipe are squeezed together during thermal expansion, and the guide tube deforms and squeezes the bellows, which is also a fatal problem. Incidentally, the arrow on the expansion joint cylinder is really not a decoration, it is a medium flow direction sign, and you can clearly see the flow direction of the pipe when installing it.
Tie rods and nutsThe adjustment is even more difficult on the scene. The function of the tie rod is to restrict the unexpected deformation of the expansion joint during transportation and installation. However, after some on-site installation, the locking nut is tightened and not loosened, which is equivalent to making the expansion joint a rigid body, and the displacement of the bellows is not exerted at all. In turn, some installation teams loosen the adjustment nut at will, the bellows extended freely under pressure, and the displacement of the pipe system was completely out of control. The nut of the adjustment rod must be according to the design requirements of the drawing. The loose one should be loosened in place, and the lock should be locked. There is no such thing as feeling.
Several pits in installation and use, everyone who stepped on them shut up
Let's startPre-displacement (cold tight)。 Some working conditions require cold tightening, so that the expansion joint is in a stretched or compressed state in advance when it is cold, and just returns to the middle position when it is hot. Here comes the question – does the amount of cold tightening count? How to calculate? This is not determined by patting the head, but it should be calculated backward according to the pipeline installation temperature, working temperature and actual stiffness of the expansion joint. When I returned to a project site, the construction team didn't do cold tightening because it was troublesome. As a result, when the pipeline was running in hot state, the bellows of the expansion joint was over-compressed, and the corrugations killed each other, resulting in direct instability.
AgainSpacing between fixed bracket and guide bracket。 The fixed bracket fixes the displacement distribution of the pipe system, and the guide bracket prevents the transverse instability of the pipe when it is displaced in the axial direction. If the spacing is large, the pipeline will sag and bend laterally between the guide frames; The set is small, the bracket is too dense, and the cost is unnecessary. There is a recommended formula in the specification, but it is no different from a blind man touching an elephant if you directly set the manual without looking at the pipe diameter or running the stress calculation.
A steam pipeline of a chemical plant, with a diameter of DN400, a working pressure of 1.6MPa and a temperature of 250℃, is equipped with a high-temperature axial expansion joint. As a result, the construction party installed the expansion joint in the opposite direction-the direction of the guide tube is opposite to the flow direction of the medium. Less than one year after operation, the inner wall of the bellows was washed out of two deep trenches by steam, and the wall thickness was thinned by more than 50%. The site leaked and stopped. When repaired, I removed it and saw that a gap had been punched out of the outlet end of the guide tube. If the direction is wrong, no matter how good the material is, it will be useless.
Give you a list of on-site inspections, just follow it:
- Are there any noticeable corrosion spots, cracks, or scratches on the bellows surface?
- Is there any abnormal vibration sound in the guide tube, and is the direction of inlet and outlet correct?
- Tie Rod Nut Status: Is the locking nut loose? Has the adjustment nut position changed?
- Is there any abnormal lateral displacement or torsion of the expansion joint?
- Is the insulation damaged and is the bellows exposed to the external environment?
- Compare the displacement indication of the expansion joint before and after the inspection. Is it beyond the design range?
What if the axial displacement is not enough?
Some working conditions are tricky, and the axial displacement is so large that you doubt your life. For example, a 500-meter-long thermal pipeline, with a temperature difference of 150℃, calculates that the expansion amount is almost 600mm. SingleAxially moving metal expansion jointEven if you are full, you can't swallow such a large displacement.
What about that? The scheme has three paths.
First, changeExternal pressure single axial expansion jointIncrease the number of ripples appropriately. With more wave numbers, the single wave displacement is small, the fatigue life is beautiful, and the total displacement can be piled up. However, the number of corrugations is not the more, the better. Too many waves will cause column instability, so we have to add a guide frame to constrain it.
Second, usingCompound hinge transverse expansion joint。 This expansion joint does not absorb axial displacement itself, but the two hinge expansion joints are used together to absorb axial elongation equivalently through transverse deformation of the pipe. It is equivalent to converting axial displacement into lateral displacement, which is digested by using the bending space of the pipe. Suitable for situations where there are natural turns in the pipeline direction.
Third, it is combined into a gimbal hinge system. Two or three hinge-type expansion joints and intermediate connecting tubes are used to form a universal hinge group, which can absorb displacement in any direction of space. This scheme is more friendly to the pipe system stress distribution, but the design and installation difficulty are up a step.
Which path to choose, in the final analysis, depends on your pipe layout and the position of the fixing bracket. After calculating the pipeline thrust, bellows stiffness and displacement, after calculating it back, the support force and equipment interface load are all right before you dare to make a plan. There is no standard answer to the selection of expansion joints, only the most suitable solution.